The growth, maintenance and regeneration of neurons is regulated at least in part by certain polypeptide growth factors, known as neuroptrophins (NTs), which bind to and activate cell surface receptors of the Trk family having an intrinsic tyrosine kinase activity. Upon neurotrophin binding, these receptors are believed become autophosphorylated on one or more amino acid residues and subsequently associate with intracellular molecules important for signal transduction (For a review, see Ulrich & Schlessinger, Cell 1990, 61:203-212; Chao, Neuron 1992, 9:583-593)
Neurotrophins are small (approx. 13 kDa) highly basic dimeric proteins that profoundly affect the development of the nervous system in all vertebrates' species.
Nerve growth factor (NGF) is the first and best characterized member of the neurotrophin family. It exists within the CNS as a homodimer, and its gene is located on chromosome 1 (p21-p22.1 region). NGF was shown to promote survival of primary sensory neurons, and sympathetic and cholinergic neurons of the basal forebrain. It has also been demonstrated that NGF is a protection factor against axotomy-induced neurodegeneration and age-related atrophy. It has been suggested that NGF plays a role in pathophysiology and pharmacotherapy of neurodegenerative disorders such as Alzheimer's disease. In addition, it was shown to significantly decrease neurodegeneration in hippocampal neurons exposed to glutamate at toxic concentrations and preserve cell morphology (reviewed by Salehi et al., 2004; Tuszynski M H, et al. Nat. Med. 2005 June; 11(6):551-5; Jakubowska-Dogru E, Gumusbas U Neurosci Lett. 2005 Jul. 1; 382(1-2):45-50; Walz R, et al Neurochem Res. 2005 February; 30(2):185-90; Hu Z et al. Neurobiol Dis. 2005 February; 18(1):184-92).
Brain-derived neurotrophic factor (BDNF) is another well-characterised member of the neurotrophin family. The BDNF gene is located on chromosome 11, band p13. The structure of BDNF protein is similar with the structure of NGF. BDNF is more widespread in the CNS than NGF. Like NGF, BDNF is abundant in the hippocampus, brain region which maintains high degree of neural plasticity in adult brain, which essentially associated with learning and memory. It is noteworthy that the regulation of expression of both NGF and BDNF within the hippocampus is, at least partly, controlled by the cholinergic and glutamatergic systems. Hippocampal damage has been shown to lead to up-regulation of BDNF expression. In addition, BDNF, similarly to NGF, acts as a survival factor for primary sensory and cholinergic neurons of the basal forebrain. Further, BDNF was found to promote survival of a wide range of other neuronal cell types, e.g. dopaminergic neurons of the substania nigra, cerebellar granule neurons, motoneurons, neurons of the locus ceruleus and retinal ganglion cells, in which NGF does not seem to play a vital role.
Neurotrophin-3 (NT-3) has a highly similar structure to both NGF and BDNF, however, the expression of this growth factor in the nervous system is somewhat different from NGF or BDNF. In contrast to NGF and BDNF the level of NT-3 in the CNS is high during foetal development and in newborn brain and it is reduced in the adult brain. NT-3 level in the hippocampus of newborn animals is significantly higher than the level of other neurotrophins. Based on these findings it was proposed that NT-3 plays a central role in early neuronal development and malfunction of this neurotrophin during development of the brain may lead to hippocampal pathologies such as schizophrenia. Like NGF and BDNF, NT-3 prolongs survival of primary sensory neurons, in particular cells of the neural crest, and it enhances dopaminergic neurons survival (for review see Shoval and Weizman, 2005).
The fourth described member of the neurtophin growth factor family is neurotrophin 4/5 (NT-4/5). NT-4/5 is as all the other neurotrophins a potent survival factor for neuronal cells, in particular it has been shown to promote survival sensory neurons of the neural crest and placodes, motoneurons, neurons of the basal forebrain and locus ceruleus. It also serves as a differentiation factor for basal forebrain neurons and motoneurons. It is involved in the promotion of nerve regeneration and synaptic activity of hippocampal neurons (for review see Shoval and Weizman, 2005).
Nerve growth factor receptor (NGFR), also referred to as p75NTR due to its molecular mass, is the ubiquitous receptor for all neutrophins. At the time of its discovery, NGFR was considered a unique type of protein. Subsequently, however, a large superfamily of tumor necrosis factor receptors were found to share the overall structure of NGFR (4 extracellular ligand-binding, cysteine-rich repeats, or CRs, and signaling through association with, or disassociation from, cytoplasmic interactors). The identification of this superfamily helped elucidate some of the biologic functions of NGFR, including its ultimate involvement in the nuclear factor kappa-B and apoptosis pathways. NGFR/p75NTR is primarily associated with cell death. As a monomer, NGFR binds all NTs with low affinity. Higher affinity binding of NTs is achieved by association the factors with higher molecular mass receptors of the tropomyosin receptor kinases family (TRK family), TRKA (NTRK1), TRKB (NTRK2), and TRKC (NTRK3). TRKA, TRKB, and TRKC are specific for NGF, NT-4/5 and BDNF, and NT-3, respectively. NT-3 also binds to TRKA and TRKB, but with significantly lower affinity (for review NTs and its receptors see Bothwell, Science 272: 506-507, 1996, Carter and Lewin, Neuron 18: 187-190, 1997, Bibel and Barde, Genes Dev. 14: 2919-2937, 2000).
Binding NTs to their receptors and regulation of this binding is very complex and strongly regulated during development and in the adult. Neurotrophins and their receptors are involved both in regulation of normal function of the nervous system and in pathology. It has been long apparent that development new compounds that are capable to enhance and/or inhibit function of neurotrophins and/or activity of their receptors would be beneficial in terms of development of new medicine for treatment of a huge number of neural system related diseases. However, despite this long felt need, such compounds have been elusive at best. As large-molecules, the therapeutic delivery of effective levels of neurotrophins themselves presents considerable, possibly insurmountable, challenges. Moreover, natural neurotrophins may interact with other receptors, such as the p75 receptor in neurons, which is associated with neuronal apoptosis and growth cone collapse. However, previous efforts to design peptidomimetic agonists and/or antagonists of Trk-receptors have also been unsuccessful. For example, cyclic peptides derived from loop 1 of the neurotrophin NGF have been reported to moderately mimic the survival activity of NGF. However, these peptides appear to function in a p75, rather Trk-receptor, dependent manner (Long et al., J. Neurosci. Res. 1997, 48:1-17). Some NGF loop 4 cyclic peptides are said to show NGF-like survival activity that is blocked by a Trk antagonist. However, the maximal survival response induced by those peptides is reported to be only 10-15% of the maximal response promoted by the NGF neurotrophin itself (Xie et al., J. Biol. Chem. 2000, 275: 29868-29874; Maliartchouk et al., J. Biol. Chem. 2000, 275: 9946-9956).
Bicyclic and tricyclic dimeric versions of BDNF loop 2 peptides have been shown to have BDNF-like activity. Again, however, the maximal survival response they induce is reported to be only 30% of the maximal response promoted by the natural neurotrophin. (O'Leary et al., J. Biol. Chem. 2003, 278:25738-25744). There continues to exist, therefore, a long felt need for compositions that can modulate (i.e., increase or inhibit) neuronal growth and recovery. There also exists a need for processes and methods (including therapeutic methods) that effectively modulate neuronal growth and recovery.